An internal combustion engine is known which comprises a plurality of power cylinders each having intake and exhaust valves which are made operative or inoperative depending upon the load conditions of the engine. Under high load conditions of the engine, the intake and exhaust valves of all the power cylinders are maintained operative and, under medium to low load conditions of the engine, the intake and exhaust valves of one or more power cylinder are made inoperative under the control of an intake/exhaust valve assembly. An example of such an intake/exhaust valve assembly is shown in Japanese Provisional patent publication No. 54-36415. The intake/exhaust valve assembly therein shown is however not fully acceptable for its disproportionately intricate and large-sized construction which requires a considerably high production cost. The present invention aims at provision of an improved intake/exhaust valve assembly eliminating such drawbacks of a prior-art intake/exhaust valve assembly.